Composition for carbon dioxide absorption

The present invention relates to a composition for absorbing carbon dioxide.

Recently, climate change accompanied by global warming becomes a matter of concern.

It is said that the greenhouse gas most influential in global warming is carbon dioxide, and techniques for recovering carbon dioxide have been actively studied.

As one of the techniques for recovering carbon dioxide, ionic liquids have been receiving attention because of their large carbon dioxide absorption amounts.

For example, Patent Literature 1 discloses an acidic gas absorption solution containing an ionic liquid containing a cation and an anion, wherein the anion is a carboxylate ion, at least one of the cation or the anion is an ion having a hydroxyl group, the carboxylate ion includes a matrix which is a non-substituted or substituted, saturated or unsaturated hydrocarbon or heteroatom-containing hydrocarbon, and the heteroatom is at least one atom selected from the group consisting of an oxygen atom, a sulfur atom, and a phosphorus atom.

However, the ionic liquid has insufficient thermal stability and is decomposed under a high temperature environment, leading to a reduction in carbon dioxide absorption amount.

The present invention has been made in consideration of the above-mentioned problems. An object of the present invention is to provide a composition for absorbing carbon dioxide which enables a suppression in decomposition of an ionic liquid even under a high temperature environment, and can maintain the carbon dioxide absorption amount.

The present inventors, who have conducted extensive research to solve the above-mentioned problems, have gained knowledge that when the ionic liquid is placed under a high temperature environment, an SN2 reaction (nucleophilic attack to the cation by the anion constituting the ionic liquid) occurs to decompose the ionic liquid.

The present inventors, who have conducted further extensive research, have found that in a composition for absorbing carbon dioxide containing an ionic liquid containing a specific cation and a specific anion and a specific protic compound in a specific molar amount ratio, the SN2 reaction can be suppressed even when the composition is placed under a high temperature environment, and therefore decomposition of the ionic liquid can be suppressed, and the carbon dioxide absorption amount can be maintained. Thus, the present invention has been completed.

Specifically, the present invention relates to a composition for absorbing carbon dioxide including an ionic liquid (A) containing a cation and an anion, and a protic compound (B) having a relative permittivity at 25° C. of 20 or more, wherein the cation at least includes a cation represented by the formula (1), the anion at least includes an anion where an acid dissociation constant (pKa) at 25° C. of its conjugate acid in water is 4.5 or more, and the ratio of a value obtained by multiplying the number of hydroxyl groups of the protic compound (B) by the number of moles of the protic compound (B) to the number of moles of the ionic liquid (A) [(number of moles of protic compound (B)×number of hydroxyl groups of protic compound (B))/number of moles of ionic liquid (A)] is 0.2 to 1.0.

[wherein Rand Reach independently represent a hydrogen or a C1-C6 linear alkyl group.]

The present invention can provide a composition for absorbing carbon dioxide which enables a suppression in decomposition of an ionic liquid even under a high temperature environment, and can maintain the carbon dioxide absorption amount.

The composition for absorbing carbon dioxide according to the present invention is a composition for absorbing carbon dioxide including an ionic liquid (A) containing a cation and an anion, and a protic compound (B) having a relative permittivity at 25° C. of 20 or more, wherein the cation at least includes a cation represented by the formula (1), the anion at least includes an anion where an acid dissociation constant (pKa) at 25° C. of its conjugate acid in water is 4.5 or more, and the ratio of a value obtained by multiplying the number of hydroxyl groups of the protic compound (B) by the number of moles of the protic compound (B) to the number of moles of the ionic liquid (A) [(number of moles of protic compound (B)×number of hydroxyl groups of protic compound (B))/number of moles of ionic liquid (A)] is 0.2 to 1.0.

In the composition for absorbing carbon dioxide according to the present invention, an SN2 reaction (nucleophilic attack to the cation by the anion constituting the ionic liquid) can be suppressed even when the composition is placed under a high temperature environment, and therefore decomposition of the ionic liquid (A) can be suppressed, and the carbon dioxide absorption amount can be maintained.

<Ionic Liquid (A)>

The composition for absorbing carbon dioxide according to the present invention contains an ionic liquid (A).

The ionic liquid (A) contains a cation and an anion.

The cation at least includes a cation represented by the formula (1):

wherein Rand Reach independently represent a hydrogen or a C1-C6 linear alkyl group.

Examples of the cation specifically include 1-methylimidazolium, 1-ethylimidazolium, 1-propylimidazolium, 1-butylimidazolium, 1,3-dimethylimidazolium, 1-ethyl-3-methylimidazolium, 1-propyl-3-methylimidazolium, 1-butyl-3-methylimidazolium, 1-hexyl-3-methylimidazolium, and 1,3-diethylimidazolium.

Among these, 1-ethyl-3-methylimidazolium, 1-butyl-3-methylimidazolium, and 1,3-diethylimidazolium are preferred, and 1-ethyl-3-methylimidazolium and 1-butyl-3-methylimidazolium are also preferred from the viewpoint of the initial carbon dioxide absorption amount (the carbon dioxide absorption amount immediately after the composition for absorbing carbon dioxide according to the present invention is prepared).

The cation may contain a cation other than the cation represented by the formula (1).

Examples of the cation other than the cation represented by the formula (1) include cations having a hydroxyl group. From the viewpoint of the carbon dioxide absorption amounts [initial carbon dioxide absorption amount (immediately after the composition for absorbing carbon dioxide according to the present invention is prepared) and that under a high temperature environment], the proportion of the weight of the cation having a hydroxyl group relative to that of the cation represented by the formula (1) is preferably 10% by weight or less, more preferably 5% by weight or less. Particularly preferably, the cation having a hydroxyl group is not contained.

The amount of the cation other than the cation represented by the formula (1) is preferably 10 mol % or less relative to the total amount of the cations.

The anion at least includes an anion where an acid dissociation constant (pKa) at 25° C. of its conjugate acid in water is 4.5 or more.

Examples of the anion specifically include a carboxylate ion such as a saturated aliphatic carboxylate ion, an unsaturated aliphatic carboxylate ion, or an aromatic carboxylate ion. Among these, preferred is an anion represented by the formula (2), namely, a saturated aliphatic carboxylate ion from the viewpoint of the carbon dioxide absorption amount.

In the formula (2), Rrepresents a C1-C5 linear alkyl group.

The acid dissociation constant (pKa) indicates the acid dissociation constant (pKa) in an aqueous solution, for example, according to Kagaku Binran (II) (revised 4th edition, 1993, edited by The Chemical Society of Japan, Maruzen Company, Limited). A smaller value thereof indicates higher acid strength. Specifically, the acid dissociation constant pKa in an aqueous solution can be actually measured by measuring the acid dissociation constant at 25° C. using an infinite dissolution.

The carboxylic acid as a carboxylate ion source is preferably a saturated aliphatic carboxylic acid.

Examples of the saturated aliphatic carboxylic acid include acetic acid (pKa=4.6), propionic acid (pKa=4.7), butyric acid (pKa=4.8), isobutyric acid (pKa=4.9), valeric acid (pKa=4.9), pivalic acid (pKa=5.0), hydrangelic acid (pKa=4.9), and isovaleric acid (pKa=4.9).

The carboxylic acid as the carboxylate ion source is preferably acetic acid and propionic acid, more preferably acetic acid from the viewpoint of the initial carbon dioxide absorption amount (the carbon dioxide absorption amount immediately after the composition for absorbing carbon dioxide according to the present invention is prepared).

The anion is preferably acetate or propionate, more preferably acetate.

The carboxylic acid as the carboxylate ion source may be an unsaturated aliphatic carboxylic acid or an aromatic carboxylic acid.

Examples of the unsaturated aliphatic carboxylic acid include crotonic acid (pKa=4.7).

Examples of the aromatic carboxylic acid include 4-butoxybenzoic acid (pKa=4.5).

Besides of the anion described above, an anion having a hydroxyl group may be contained. From the viewpoint of the initial carbon dioxide absorption amount [initial carbon dioxide absorption amount (immediately after the composition for absorbing carbon dioxide according to the present invention is prepared) and the carbon dioxide absorption amount under a high temperature environment], the proportion of the weight of the anion having a hydroxyl group relative to that of the anions contained in the composition for absorbing carbon dioxide is preferably 10% by weight or less, more preferably 5% by weight or less. Particularly preferably, the anion having a hydroxyl group is not contained.

Examples of the carboxylic acid forming the anion having a hydroxyl group include 3-hydroxypropionic acid (pKa=4.5).

The anion may contain an anion where the acid dissociation constant (pKa) at 25° C. of its conjugate acid in water is less than 4.5.

From the viewpoint of the initial carbon dioxide absorption amount [initial carbon dioxide absorption amount (immediately after the composition for absorbing carbon dioxide according to the present invention is prepared) and the carbon dioxide absorption amount under a high temperature environment], the proportion of the weight of the anion where the acid dissociation constant (pKa) at 25° C. of its conjugate acid in water is less than 4.5 relative to the weight of the anion where the acid dissociation constant (pKa) at 25° C. of its conjugate acid in water is 4.5 or more is preferably 10% by weight or less, more preferably 5% by weight or less. Particularly preferably, the anion where the acid dissociation constant (pKa) at 25° C. of its conjugate acid in water is less than 4.5 is not contained.

The amount of the anion where the acid dissociation constant (pKa) at 25° C. of its conjugate acid in water is less than 4.5 is preferably 10 mol % or less relative to the total amount of the anions.

The amount of the anion other than the anion represented by the formula (2) is preferably 10 mol % or less relative to the total amount of the anions.

Examples of carboxylic acids where the acid dissociation constant (pKa) at 25° C. in water is less than 4.5 include benzoic acid (pKa=4.2), glycolic acid (pKa=3.8), difluoroacetic acid (pKa=1.3), dichloroacetic acid (pKa=1.4), and fluoroacetic acid (pKa=2.6).

The ionic liquid (A) can be prepared by a known method, and the optimal conditions can be selected according to the raw materials. Examples of the known method include a method of acting a carboxylic acid as the carboxylate ion source with a cation.

From the viewpoint of high carbon dioxide absorption performance and suitable suppression of decomposition of the ionic liquid (A), the ionic liquid (A) is preferably 1-ethyl-3-methylimidazolium acetate, 1-butyl-3-methylimidazolium acetate, or 1,3-diethylimidazolium acetate, and is also preferably 1-ethyl-3-methylimidazolium acetate or 1-butyl-3-methylimidazolium acetate.

The weight of the ionic liquid (A) is preferably 75% by weight or more and 98% by weight or less relative to the total weight of the composition for absorbing carbon dioxide according to the present invention.

When the weight of the ionic liquid (A) falls within the range above, the carbon dioxide absorption performance can be suitably imparted to the composition for absorbing carbon dioxide according to the present invention.

The weight of the ionic liquid (A) is more preferably 86% by weight or more and 95% by weight or less relative to the total weight of the composition for absorbing carbon dioxide according to the present invention.

<Protic Compound (B)>